Cape fur seals.
When droves of dead Cape fur seal pups washed up at a colony in Lamberts Bay in September, Dr Tess Gridley and her small team quickly took action, freezing seven of the freshest carcasses. At other sites, they encouraged permitted scientists to follow suit.
These samples, “frozen in time”, are capsules of the environmental conditions the mothers were exposed to when they were in their third trimester of pregnancy, according to Gridley, the co-director of the non-profit organisation Sea Search and an extraordinary lecturer in the department of botany and zoology at Stellenbosch University.
They can help provide clues as to whether a naturally occurring marine biotoxin called domoic acid, which is produced by certain types of algae, is linked to the deaths of thousands of Cape fur seals at colonies along the entire West Coast over the past six months — and to the Cape fur seal “abortion storm” in Namibia in 2020.
With support from international advisers, Gridley’s team is now processing these carcasses to investigate whether domoic acid exposure, potentially accumulated in prey, could help to explain the die-off. This work is complicated, Gridley said, because the acid itself breaks down quickly, making identification in body samples tricky.
Previous research has demonstrated that domoic acid can lead to abortions and deaths in seal pups. In the US, domoic acid has been linked in peer-reviewed scientific studies to sea lion dying in California; in humans, it can cause amnesic shellfish poisoning.
On Thursday, the department of forestry, fisheries and the environment said an investigation had shown that the deaths were the result of a variety of factors, including malnutrition.
The Western Cape department of agriculture’s state veterinary laboratory conducted post mortems on pups and sub-adults after the Cape fur seal deaths, finding evidence of moderate to severe malnutrition.
Clinical signs received, including convulsions seen in the video footage, suggest possible biotoxin poisoning, it said. “As a result, extensive testing for significant parasites and viruses, including distemper, as well as bacterial infections were conducted on all carcasses. The results have proven either negative or insignificant.’’
All results for biotoxins were also negative, the department said. Due to the neurological signs witnessed, similar tests will be conducted on adult carcasses as “part of a more thorough, well-rounded investigation by the state vet”.
“In my opinion, this isn’t purely malnutrition,” Gridley told the Mail & Guardian. “I strongly suspect that the biotoxin, domoic acid, is responsible for this outbreak, although this is yet to be confirmed. It’s quite difficult because there isn’t actually testing available for domoic acid in South Africa, and for that reason we’re having to source test kits from overseas and … we’ve been trying to fundraise for that.”
The plate readers that are needed to identify domoic acid cost R50 000 to R100 000 and the kits for tests cost about R12 000 a batch. “There is a large cost associated with this that we are now having to find the money for,” Gridley said. “There hasn’t really been somebody in the government who has really led this so we’ve taken on this responsibility and we will carry on investigating this…
“We are seeing animals that are very thin, but it would be expected if it is domoic acid, because what happens is it can change the behaviour of the animals and it can affect their foraging efficiency,” Gridley added. “So that could explain why the animals we’ve seen who have died are so thin. We’re looking at animals with a blubber layer a quarter as thick as what it should be, including in adults, so that’s quite shocking. They definitely are very emaciated, but we think that is a response to a biotoxin, or perhaps the thinner animals are more affected and impacted.”
The mass mortalities, she said, are not purely related to overfishing. “Of course, we do have those issues and that can make the animals more susceptible to die-offs … We think maybe there are several things going on all at once …
“We’ve had to get necropsies done as quickly as possible and this has involved removing brains, which is quite specialised. For domoic acid, the testing that we want to [do] involves stomach content analysis, because [of] the acid and the algae that would produce it, which we know is around South African waters,” Gridley explained.
“So we need to look at those stomachs in detail for the presence of domoic acid and the presence of the harmful algae and then the prey the animals have eaten,’’ she said, explaining it, too, can lead to sporadic, epileptic-like seizures in animals. “It will take us some time to get to the bottom of all of this.’’
According to the department, the die-off coincided with the period when seals prepare for their breeding season and colonies see an influx of mature individuals. Adverse weather, such as strong winds and storms, in September and October had previously resulted in large numbers of pups washing up dead or emaciated on beaches adjacent to breeding colonies, it said.
Last year’s Cape fur seal die-off is the first of its kind since a similar occurrence in 1994-95. “National, provincial and local government, working with agencies and partners, will continue to monitor the situation. Any carcasses found in future will be taken to the relevant provincial state veterinary laboratory for testing. The department has issued a research permit allowing for further sampling and testing,” the department of environment, forestry and fisheries said in its statement.
[/membership]